1. Field of the Invention
The present invention relates to a dry toner for developing an electrostatic image in electrophotography, electrostatic recording, electrostatic printing and the like, a developer including the dry toner, and a process cartridge, an image forming method using the developer and an image forming apparatus using the developer. More particularly the present invention relates to a dry toner for use in copiers, laser printers and plain-paper facsimile machines and similar devices which use a direct or indirect electrophotographic development method, a developer, and a process cartridge, an image forming method and an image forming apparatus using the toner.
2. Discussion of the Background
Electrostatic latent images, which are formed on an image bearing member using a method such as electrophotography, electrostatic recording and electrostatic printing are developed with a toner in order to be visualized.
For example, visual images are typically formed as follows:    (1) a latent electrostatic image is formed on an image bearing member such as photoreceptors (latent image forming process);    (2) the latent electrostatic image is developed with a developer including a toner to form a toner image on the image bearing member (developing process);    (3) the toner image is transferred onto a receiving material, such as paper, optionally via an intermediate transfer medium (transfer process);    (4) the toner image on the receiving material is fixed upon application of heat, etc. to form a hard copy (fixing process).
As a developer for developing an electrostatic image formed on the surface carrying a latent image thereon, there are known two component developers containing a carrier and a toner, and single component toners requiring no carrier (a magnetic toner and a non-magnetic toner)
Conventional dry toners for use in electrophotography, electrostatic recording and electrostatic printing are typically prepared by fusing/kneading toner binders such as styrene-containing resins and polyesters with a colorant and so on followed by finely pulverizing.
(Problems with Reference to Fixability)
These toners are fixed by heating and fusing with a heat roll after the toners are developed and transferred onto a medium such as paper. When the temperature of the heat roll is too high during fixing, the toner is excessively fused and adhered to the heat roll too much, resulting in occurrence of a hot offset problem. When the temperature of the heat roll is too low to sufficiently fuse the toner, there is a problem in that the toner is inadequately fused and thereby fixing is insufficient.
In light of saving energy and miniaturizing devices such as copiers, a toner is desired which has a high hot offset temperature (i.e., high hot offset resistance) and has a low fixing temperature (i.e., good fixability at a low fixing temperature). In addition, the toner is required to be heatproof so as not to cause blocking when the toner is in storage and is used at an atmospheric temperature in the device in which the toner is installed.
(Problems with Reference to Particle Diameter and Shape)
Toner particle diameters become smaller and smaller in order to improve images by goving high quality and high resolution. However, a toner that is manufactured by an ordinary kneading and pulverization method has an irregular shape. Such a toner is fractured in a machine when the toner is stirred with a carrier or contacts with a developing roller, a toner furnishing roller, a layer regulating blade and a triboelectrical charging blade. Therefore extremely fine particles are generated and the fluidizer on the surface of the toner is buried in the toner, resulting in deterioration of image qualities. In addition, due to its irregular shape, fluidity of the toner is so bad that a large amount of fluidizer has to be included therein, and the toner has a large volume when the toner is filled in a toner container, which is a barrier to miniaturization.
Furthermore, since the process for transferring toner images from a photoreceptor to an intermediate transfer medium or a transfer medium becomes complicated, problems occur such as image omission due to poor transferability stemming from irregularity in the shape of pulverized toners and an increase of toner consumption to compensate the image omission.
Therefore, there is an increasing demand for further improvement in transfer efficiency in order to reduce the amount of toner consumption, obtain high definition images without omission and lower running cost. If transfer efficiency is extremely excellent, it is unnecessary for an image forming apparatus to have a cleaning unit removing untransferred toner from a photoreceptor or a transferring medium. At the same time, there are other merits such as miniaturization of machines, low running cost and no waste toner. In order to avoid the problems arisen from irregularity in shape of the toner, various kinds of spherical toners have been proposed.
Among the toners, the following toners have been proposed particularly for improving high temperature resistance. For example, (1) a toner including a polyester as a toner binder which is partially cross-linked by multifunctional monomers is proposed in published unexamined Japanese Patent Application No. (hereinafter referred to as JOP.) 57-109825 and (2) a toner including a urethane modified polyester as a toner binder is proposed in JOP. 7-101318. In addition, (3) a full color toner prepared by granulating fine polyester particles and fine wax particles is proposed in JOP. 7-56390 in order to reduce the amount of an oil which is applied to a heat roll.
Further, in order to improve powder fluidity and transferability of a toner having a small particle, there have been proposed: (4) a toner polymerized by suspension polymerization after dispersing a vinyl monomer composition including a colorant, a polar resin and a releasing agent in water (JOP. 9-43909); and (5) a spherical toner obtained by granulating a toner, which includes a polyester resin and is dispersed in a solvent, in water (JOP. 9-34167).
In addition, there is disclosed (6) a substantially spherical dry toner made of a polyester resin which is modified by urea bonding in JOP. 11-133666.
However, the toners disclosed in (1) to (3) have such insufficient fluidity and transferability that it is very difficult to obtain quality images even when the toners have a small particle. Further, the toners disclosed in (1) and (2) are not suitable for practical use because of not having a good combination of high temperature preservability and low temperature fixability and because of producing images having unsatisfactory gloss when used as a full color toner. In addition, the toner disclosed in (3) is insufficient in low temperature fixability and further is not satisfactory in the light of hot offset resistance for oil-free fixing. The toners disclosed in (4) and (5) have improved fluidity and transferability. However, the toner disclosed in (4) requires large fixing energy due to its insufficient low temperature fixability. This problem is apparent especially when the toner is used as a full-color toner. The toner disclosed in (5) is superior in low temperature fixability but insufficient in hot offset resistance so that it is inevitable to apply oil to a heat roll when the toner is used as a full-color toner.
The toner disclosed in (6) can produce images having high gloss while having good releasability when used as a full-color toner because viscoelasticity of the toner can be adjusted by using a polyester elongated by urea bonding. Especially the toner disclosed in (6) is effective in preventing images so-called electrostatic offset in that toner images scatters or adheres to a fixing roller when the fixing roller is statically charged. The toner disclosed in (6) can reduce a chance of such toner scattering or adhesion due to electrical neutralization between positive chargeability created by the urea bonding portions of the polyester resin and weak negative-chargeability of the polyester resin per se.
Although the toner has the advantages mentioned above, the toner is fractured in an image forming apparatus when the toner is stirred with a carrier or contacts with a developing roller, a toner furnishing roller, a toner layer regulating blade and a triboelectrically charging blade. Thereby, extremely fine particles tend to be generated and a fluidizer on the surface of the toner is buried in the toner. This results in deterioration of image qualities and a shortening of toner life.
Because of these reasons, a need exists for a dry toner having a small particle diameter, a high electric resistance, a long life and having excellent powder fluidity, transferability and high temperature resistance.